That was true in the early days of the format, but these days a significant number of discs only have a 5.1 soundtrack. They can safely do that because the DVD format requires that players include the ability to downmix 5.1 to stereo for the analog outputs. Early on, there were concerns that this downmix (which at the time was the output that would most often be used, since Dolby Digital receivers were still a significant premium) was not as effective as a studio-mixed two-channel track. As a result, a Dolby 2.0 track was regularly used for the first few years of the format and was very often the default track.